Fix compiling warnings

The patch removes the following warnings:

system/vold/vdc.c: In function 'do_cmd':
system/vold/vdc.c:71:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
[...]

In file included from system/vold/Fat.cpp:34:0:
bionic/libc/kernel/common/linux/fs.h:109:0: warning: "MS_RMT_MASK" redefined [enabled by default]
In file included from system/vold/Fat.cpp:31:0:
bionic/libc/include/sys/mount.h:64:0: note: this is the location of the previous definition
[...]

Change-Id: I1e950dcec87f8bae0cec61a1611126b3abc0c90e
diff --git a/vdc.c b/vdc.c
index 7dad143..59f34d6 100644
--- a/vdc.c
+++ b/vdc.c
@@ -57,7 +57,7 @@
 static int do_cmd(int sock, int argc, char **argv) {
     char final_cmd[255] = "0 "; /* 0 is a (now required) sequence number */
     int i;
-    int ret;
+    size_t ret;
 
     for (i = 1; i < argc; i++) {
         char *cmp;